Enceladus’ south pole is warmer than expected

An article published in the journal “Nature Astronomy” describes the discovery that the south pole of Enceladus, one of the planet Saturn’s moons, is warmer than expected under the icy surface. A team of researchers led by Alice Le Gall of LATMOS and UVSQ studied detections carried out by the Cassini space probe during a flyby in 2011 concluding that the underground ocean on Enceladus is closer to the surface than previously thought.

Over the years, the Cassini space probe gathered a growing body of evidence of an underground ocean on Enceladus. According to a research published in September 2015, this ocean is even global and this possibility helped to keep up the interest in this moon and the possibility that under its frozen surface life forms emerged.

The Cassini mission included a series of Enceladus flybys that aimed to gather more data to better understand its internal structure and the hydrological and geological activities taking place inside the moon. In scientific missions, the data remain available to scientists who can keep on studying them even years away. That’s exactly what happened with the data collected during a flyby occurred in 2011.

Unfortunately on that occasion the only high-resolution detections of the south pole of Enceladus at microwave wavelengths were carried out. As a result, there are limited data that still gave a surprise because the surface area examined was found to be particularly warm.

The term warm should be understood in a very relative sense because the temperatures detected a few meters below the surface are between 50 and 60 Kelvin – in comparison the Earth’s south pole is incandescent! – but are still up to 20 Kelvin higher than other areas under Enceladus’ surface.

The researchers verified the possible causes of the abnormal heating and ruled out solar illumination and Saturn’s influence. The area affected by the anomaly is arc-shaped, approximately 500 kilometers (310 miles) long and 25 kilometers (15 miles) wide at a distance between 30 and 50 kilometers (between 18 and 30 miles) north of the active fractures called jargon tiger stripes.

The thermal anomaly is especially pronounced over three fractures similar to the tiger stripes except for the fact that they seem not active and therefore emit no jets of materials. Their existence suggests that on Enceladus there have been periods of geological activity in different areas during its past.

The observations concerned a limited region but it’s possible that the phenomenon is more extended. The polar ice crust could be only 2 kilometers (1.2 miles) thick, a hypothesis consistent with a study published in 2016. This thinner crust is more subject to the deformation caused by Saturn’s gravitational influence helping to keep the temperatures higher.

The fact that at least in an area the icy crust is thinner is also an interesting fact considering a possible mission of a space probe equipped with specific instrument to try to study the underground ocean. A crust over 20 kilometers (12 miles) thick is a problem, a 2 kilometers (1.2 miles) thick crust would make radar surveys of the subsurface easier.

Enceladus’ south pole with the area studies in the coloured band (Image NASA/JPL-Caltech/Space Science Institute; Acknowledgement: A. Lucas)